Modern aircraft are generally equipped with several means of radio communication between the pilot or other aircraft personnel and ground stations or other aircraft. For convenience of explanation the present invention is described in the context of pilot communication but persons of skill in the art will understand that it applies to any other flight crewmembers and even to passengers who may have access to communication facilities on the aircraft. Typically the pilot has several radios and several microphones that he or she may use. The pilot selects a particular microphone and radio by means of one or more switches on a control panel. The selected microphone itself is typically activated by depressing a ‘Push-To-Talk’ (PTT) switch whereupon the radio to which the microphone is coupled will transmit whatever message the pilot speaks into the microphone. The PTT may be a part of the microphone itself, as for example with a hand held mike or it may be remotely located. Pilots often wear a headset with a boom mike whose PTT switch is located on the aircraft control yoke. These are non-limiting examples of typical arrangements. The particular microphone and PTT switch arrangement is not critical.
Because of safety considerations it is important that a stuck PTT switch in a particular aircraft not preempt the particular radio channel being used, e.g., cause that aircraft's transmitter to stay on for an extended period of time, thereby preventing others sharing the same channel from communicating. A stuck PTT switch condition occurs when a PTT switch has been depressed or otherwise activated for a time exceeding a preset threshold Tc. A stuck PTT switch condition can result from several causes as, for example: (i) the pilot has held the PTT switch closed for a time exceeding Tc, or (ii) mechanical or electrical failure has occurred which prevents the PTT switch from returning to its OFF state. Typical present day avionics systems do not distinguish between these conditions. In either case, once Tc is exceeded the avionics system deactivates all of the pilot's PTT switches so that further transmission is precluded and the radio channel is cleared for use by others. The pilot's PTT switches remain disabled until the stuck-switch condition is cleared. Even when only one of the pilot's available PTT switches is ‘stuck’, for example on a hand mike, his other PTT switch, e.g. for a boom mike, is also disabled. Thus, when a ‘stuck PTT switch’ condition occurs in the prior art, all of the pilot's normal means of communication are disabled. This usually does not affect the copilot's communication capability in a dual control aircraft with duplicate microphones, PTT switches, and so forth. In addition, per RTCA/DO-207 the pilot usually has a safety over-ride switch that permits essential communication by the pilot in emergencies. This over-ride switch must be a protected switch so that it cannot be accidentally left activated. This arrangement while workable is inconvenient and more flexible means of dealing with a stuck PTT switch condition are needed.
Accordingly, it is desirable to provide a ‘stuck PTT switch’ response that allows non-stuck PTT switches and corresponding mikes to be used so that normal communications can continue on the alternative systems, despite the ‘stuck switch’ condition on one of the available mikes. In addition, it is desirable to provide this improved capability with minimum cost and little or no added complexity. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.